The present invention relates generally to radio-frequency (RF) signal filters, and, more particularly, to multi-resonator ceramic filters and mask tuning and adjusting the resonators thereof.
Conventional multi-resonator ceramic filters typically include a plurality of resonators such as foreshortened short-circuited quarter wavelength coaxial or helical transmission lines. The resonators are arranged in a conductive enclosure and may be coupled one to another by apertures in their common walls. Each resonator is commonly tuned to the desired response characteristics in one of two ways.
One way of tuning such resonators is by employing a tuning screw which inserts into a hole extending through the middles of the resonator (see U.S. Pat. No. 3,728,731). Unfortunately, the tuning screw is bulky, it requires mechanical locking elements which may offset the desired coupling between resonators, and, due to the adjustability of the screw before it is locked, it renders these filters susceptible to becoming detuned.
Another way of tuning each resonator is by plating one surface of the ceramic filter at each resonator with conductive plating material (see U.S. Pat. Nos. 4,431,977 and 4,742,562). Typically, the surface is plated between the hole in the middle of the resonator and a side wall coupled to the conductive enclosures. This plating is then abraded away for each resonator in the filter until the desired response characteristics are obtained. This approach is undesirable in that it is extremely labor intensive and therefor costly. Plating at each resonator must be repeatedly abraded followed by testing for the desired response characteristics. If too much plating is removed, the filter must be replated, backtuned, or discarded.
For these reasons, there is a need for an improved ceramic filter tuning technique which overcomes the foregoing deficiencies.